This invention relates in general to flexography and, in particular, to an apparatus for quickly removing and interchanging rollers and cylinders in a flexographic printing system.
Flexography prints from a raised image and its print plates are generally made of flexible, elastomeric materials. The section of the plate carrying the ink is the raised portion obtained by removing and lowering the nonprinting areas through cutting, molding, etching, dissolving or washing them away.
Flexographic inks are traditionally thin, highly fluid and rapid-dryings; although paste-type inks may also be used. These inks are formulated from resins which are both solvent and water reducible.
Plate materials include various moldable, natural and synthetic rubber compounds and photo-polymer materials. The plates are generally affixed to a plate cylinder with double-sided sticky tape.
Press frame designs include central impression, in-line and stack types. The central impression uses one common impression cylinder around which several printing stations are placed. The in-line design involves a tandem series of printing stations placed in a row. The stack version has individual printing stations mounted on frames one above the other in two "stacks" generally one to four on each side of a vertical frame.
The typical flexographic printing station or roller grouping from ink reservoir to substrate impression cylinder generally includes four rollers. A doctor roll turning in ink delivers ink to a steel or ceramic anilox roll, a plate cylinder and an impression cylinder.
Flexography is unique among other printing processes in that it was developed primarily for the printing of packaging materials. Since packaging materials are mostly used in roll form for feeding into form and fill, overwrapping, bag making and other continuous web processing machines, it follows that most flexographic printing is done roll to roll. Flexography has grown today beyond the printing of packaging materials and now includes a wide variety of other products.
In its simplest and most common form, the flexographic printing system consists of four basic parts: doctor roll, anilox roll, plate cylinder, and impression cylinder.
The doctor roll is generally a rubber-covered roll of either natural or synthetic rubber. It is positioned to rotate in a reservoir of thin ink. Its main purpose is to pick up and deliver a relatively heavy flow from the reservoir to the anilox roll. The anilox roll, is usually a metal or ceramic coated roll engraved over its entire surface with tiny cells numbering from 80 to over 500 per lineal inch. The purpose of the anilox roll is to supply a controlled, metered, fine film of ink to the printing plates affixed to the plate cylinder. For this reason, the doctor and anilox rolls are set to rotate under considerable pressure against each other in a wringer-like action to squeeze away excess ink from the surface of the anilox roll, leaving ink primarily in the engraved cells. The anilox roll in turn continues to rotate and ink is deposited on the raised image area of the printing plate on the plate cylinder.
The contact pressure of the anilox roll to the printing plate is set to be as light as practical in order not to over ink or cause ink to be pressed down on the shoulders of the raised image area of the plates. The anilox roll must travel at the same surface speed as the plate cylinder and is geared accordingly.
Variations of the common flexigraphic two-roll inking system described above are possible. In addition to the ink metering action between the doctor and anilox rolls, it is possible to add a "doctor blade" to shave the surface of the anilox roll just beyond the ink metering location. Its purpose is to increase the removal of surface ink and insure a more controlled inking of the printing plates. The doctor blade is usually made of spring steel or a phenolic material.
Other variations eliminate the rubber doctor roll and position the engraved anilox roll in its place. An ink applicator delivers a heavy flow of ink to the anilox roll pumped from a remote tank. The doctor blade is positioned just beyond the applicator. The ink reservoir pan serves as a catch basin funnelling ink back to the remote ink tank.
The plate cylinder is generally a steel or aluminium cylinder placed between the anilox roll and the impression roll. Printing plates are adhered to it through the use of a double-sided adhesive tape. The total plate cylinder diameter including adhesive tape and printing plate must equal the pitch diameter of the driving gear. For this reason, the bare plate cylinder for a given printing repeat length must be reduced in diameter or "undercut" to accomodate the tape and printing plate.
The anilox roll then transfers a finely metered film of ink to the raised surface of the plate, which in turn transfers the ink to the surface of the substrate or web. The impression roll is a smooth polished metal cylinder which serves to back up and support the substrate as it comes in contact with the printing plate. The surface speed of the impression cylinder with substrate must be identical to that of the plate cylinder and anilox roll. Otherwise, slurring, halos, smeared printing and reduced plate life will result.
Flexographic web-fed presses, generally consist of four basic sections, with a multiple of variations including adaptability to many in-line operations for specific purposes: (1) Unwind and infeed section; (2) printing section; (3) drying section; and (4) outfeed and rewind section, or subsequent in-line operation.
The roll of stock to be printed must be held under control so that the web can unwind into the press as needed, in proper alignment and under proper tension to prevent slack and wrinkles. The tension should be not so great that it will cause web stretching or breakage. In simplest form, this all may be accomplished by holding the roll in chucks on a steel shaft mounted in plain bearings with a manually controlled hand brake for tension control and a threaded rod and hand wheel for lateral adjustment. In prior art systems, almost continuous monitoring of the system by the operator is required.
An effective unwind and infeed system may include some or all of the following: (a) Multiple unwind positions; (b) rotating turrets to faciliate reloading; (c) Semi automatic chucking; (d) precision bearings; (e) automatic side-guide control; (f) automatic tension control with tension sensing devices; (g) driven infeed rolls; and (h) automatic roll splicing.
An unwind section may also include a nest of internally heated, steel rolls, or the rolls used for infeed tension control may be heated. The purpose for this is to "open" the surface of heavily glazed or "tight" papers by preheating, thus, making the surface more receptive to the ink. Preheating in this manner is also beneficial with some plastic materials, making them lay out flatter which reduces their tendency to wrinkle.
A single-color station comprised of the doctor roll, anilox roll, plate cylinder and impression roll is sufficient to constitute a printing station. However, the vast majority of presses are multi-color, with from two to eight printing stations in the printing section.
Some presses have their multiple units arranged in a horizontal row, each standing on the floor, similar to rotogravure, and are called "in-line" presses. Quite common in flexography is the "stack" type, with two to four colors arranged one above the other in the frame. A third type is the central impression press, where, like rotary letterpress, the color units are arranged in sequence around a common impression roll.
The drying section usually includes between-color drying capacity in order to print color-on-color, plus an after-dryer to remove the remaining solvent before winding into a roll. The most common method of drying is high velocity, heated air. There are several other methods in use, some of which require specially formulated inks. They include electron beam curing, electric infrared, ultra-violet and dielectric systems.
In prior art flexographic printing systems the doctor roll, the anilox roll, the plate cylinder and the impression roll are all securely held in place by bolts or other types of fasteners. In order to change the plate cylinder or other rolls a significant amount of time is required, for example, a half hour or more. In addition, the web must usually be stopped. Accelerations due to stopping and starting the web results in wastage of the web material. Furthermore, the cylinder and rollers must be readjusted since their relative positions are extremely critical. This has posed a severe problem in the prior art systems when "short run" printing jobs are requested. Many times it is simply uneconomical to perform "short run" printing jobs due to the wastage.
The present invention overcomes these drawbacks in the prior art.